The present invention relates to a method for making a lithographic printing plate comprising a scanning exposure of a presensitized plate having a photopolymerizable layer on a substrate having a hydrophilic surface thereon to laser light.
As a plate useful for making such a lithographic printing plate, there has widely been used a PS plate, which comprises a hydrophilic surface provided thereon with a lipophilic light-sensitive resin layer. The plate-making process for such a lithographic plate currently comprises exposing a PS plate to light rays through images such as an image-carrying lithfilm and then removing the non-image area through dissolution thereof with a developer to thus give a desired printing plate.
There has widely been used digitizing techniques in which image information is electronically processed, accumulated and output using a computer and a variety of new image-outputting techniques have been practically used in response to the development of such digitizing techniques. As the result, the computer-to-plate technique which comprises the steps of incorporation of digitized image information into highly coherent radiant rays such as laser beams and scanning-exposure of a PS plate to the radiant rays to thus directly prepare a printing plate without using any lithfilm, is desired. For this reason, recently it becomes an important technical subject to obtain a PS plate that can be suitably used for this purpose.
For the preparation of a presensitized plate which can be exposed with scanning laser beam, a PS plate having as an ink receptive photosensitive resin layer (hereinafter, referred to as photosensitive layer) made of a photopolymerizable composition which shows an excellent exposing speed, on a hydrophilic substrate has been proposed and been commercially available. The photopolymerizable composition basically consists of an ethylenically unsaturated compound, a photopolymerization initiation system (it is also called as photoinitiation system), and a binder resin. The photoinitiation system absorbs light to generate active radicals to cause addition polymerization of ethylenically unsaturated compound. Thus, the photosensitive layer becomes insolubilized to form an image. With regard to the conventional CTP system utilizing a photopolymerization composition comprising such a photoinitiation system and a light source of visible light source with a long wavelength such as Ar laser (488 nm) and FD-YAG laser (532 nm), it is desired to be written in a higher speed so that the productivity in a process to make a printing plate becomes high. However, the purpose has not been achieved since the output of the light source is not sufficient and the sensitivity of the PS plate is not sufficiently high.
Recently, a semiconductor laser that can continuously oscillate in the range between 350 nm and 450 nm, e.g., laser of InGaN type, is in the practical stage. The scanning exposure system using the light source having such a short wavelength has a merit of constructing an economical system with sufficient output since the semiconductor laser can be made at a low cost. In addition, the system has an advantage that it can be worked under a safelight having a higher lightness than the conventional system using FD-YAG and Ar laser since the PS plate has a short exposure range. In addition, it is still strongly desired in the imaging field to obtain a photoinitiation system having a high sensitivity.
The Japanese Un-examined Patent Publication (hereunder referred to as “J.P. KOKAI”) No. 2001-100412 discloses a photosensitive composition which has a photoinitiation system that is highly sensitive to the light having a wavelength of from 350 to 450 nm.
However, such a high sensitive printing plate sometimes occurs a light-fog due to the reflected light (flare light), especially when the exposure of the plate is conducted with a laser-exposing device having an inner drum system. For example, if one side of a negative-working PS plate is imagewise-exposed to a light through a solid image, a fog due to a bad developability is caused when the opposite side is a non-image area. When the opposite side (140 to 220° to the light source) is a dot area, the dot becomes widen. As the result, it is difficult to reproduce the small image or highlight portion.